The standard model of elementary particle physics I described in Chapter 1
continues to account for all experimental observations, to suggest
fruitful new observations and to serve as a point of departure for new
explorations. Many of the hopes and dreams expressed in Chapter 2 by
Sheldon Lee Glashow are now the firmly established foundations of our
understanding. Over the past several years, remarkable progress has been
made in applying quantum chromodynamics (QCD), the theory of strong
interactions, to experiments and in extracting detailed, precise
information about the high-energy interactions of quarks and gluons from
observations of the products of proton-antiproton collisions. Still more
incisive experimental studies are anticipated from the 2-TeV Tevatron
Collider at Fermilab.
Steady but less dramatic progress has been made in deducing the
consequences of QCD in the low-energy (or long-distance) regime where the
strong interactions are strong, the realm of hadron structure. In order
to deal with the existence and properties of the hadrons themselves, it
is necessary to devise a new computational strategy that does not break
down when the interaction becomes strong. The most promising approach has
been the crystal lattice formulation of QCD described in Chapter 3 by
Claudio Rebbi, in which space-time is viewed as a discrete, rather than a
continuous, structure. This makes it possible to exploit many of the
techniques developed earlier in statistical mechanics for the study of
spin systems such as magnetic materials.
One of the most valuable implementations of this strategy for studying
hadron structure uses computer simulations in which different gluon
field configurations are explored by random sampling Monte Carlo
techniques. These techniques make heavy demands on computer time and have,
in turn, spurred the development of new computer architectures. Further,
these calculations have
yielded suggestive evidence that quarks and gluons are indeed permanently
confined in QCD. The ultimate hope is to compute the spectrum and
properties of hadrons from first principles.
The search for a new state of matter, the quark-gluon plasma, which might
form when hadronic matter is compressed and heated to extreme temperatures, is beginning in earnest, using high-energy heavy-ion collisions.
Understanding what questions to ask of this new kind of experiment is an
important priority for the immediate future.
The success of the standard model challenges us to examine its
self-consistency in order to create a more complete "theory of the world."
To approach this challenge, we look to the standard model itself for
guidance. The "Open Questions" cited in Chapter 1 are among the larger
issues that guide this speculation and experimentation. The shortcomings
of the standard model may also be expressed in somewhat different form.
The sharpest issue posed by the standard model is our incomplete understanding of the scalar, or Higgs, part of the electroweak theory, that
is, the mechanism that conceals the electroweak gauge symmetry. A second
concern is the meaning of quark–lepton generations. The idea that quarks
and leptons should be grouped in generations seems necessary for the
internal consistency of the electroweak theory, but we do not know why
this should be so. Third, we may even dare to question the origin of the
gauge symmetries themselves. Such questions—and this is but a partial
list—stimulated by the standard model itself, continue to drive our desire
to find ever simpler, more general descriptions of nature.
One possible solution to the Higgs problem introduces a completely new
set of elementary particles with spins differing by one-half unit from
those of the known quarks, leptons and gauge bosons. These postulated new
particles are the consequences of a conjectured "supersymmetry" that
relates particles of integer and half-integer spin. Supersymmetry cannot be
an exact symmetry
of the world in which we live, for that would imply, for example, a spinless
counterpart of the electron with the same mass that would have been
observed long ago. Supersymmetry would stabilize the Higgs boson mass
below 1 TeV and the supersymmetric partners of the known particles would
have masses less than 1 TeV. Part of the appeal of supersymmetry is that
local form leads to quantum theories that have Einstein's gravitation as
their classical limit. This raises the possibility of obtaining a quantum
theory of gravity and of incorporating gravity into a unified description
all the fundamental interactions.
Extensive exploratory searches for superpartners at accelerators have been
negative so far. They have restricted specific models of sypersymmetry, but
have not ruled out supersymmetry as a means of resolving the problem of
electroweak symmetry breaking. Negative results become decisive only after a
thorough exploration of the 1-TeV scale, one
of the principal goals of the Superconducting Super Collider. The machines
now coming into operation will only extend the search to a few tenths of a
TeV, where the first superpartners could be found.
A second possible solution to the Higgs problem is based on the idea that
the Higgs boson is not an elementary particle. but a composite made out of
elementary constituents analogous to the quarks and leptons. Resembling the
usual quarks and leptons in certain respects, these new constituents would
have a new type of ultrastrong interaction—often called
would confine them within about 10-17 centimeters. The new phenomena would
include a rich spectrum of bound states, akin to the spectrum of known
hadrons. Although there is no evidence yet for any of these new particles,
of the technicolor scenario is that it goes beyond the Higgs mechanism of
the standard model in much the same way as the
theory of superconductivity gives a microscopic interpretation of earlier
phenomenological descriptions of the superconducting state.
Other theoretical inventions deal with the meaning of generations and the
origin of gauge symmetries. These, too, imply new kinds of matter and
force particles. A particularly audacious initiative is the search for a
"superstring" theory, described in Chapter 11 by Michael Green, which would
solve all the outstanding problems while unifying gravitation and the
other forces of nature. The observation of supersymmetric partners at or
below 1 TeV would provide powerful encouragement for
superstrings, for which supersymmetry is an essential foundation.
Theoretical speculation of this sort is helpful and often indispensable, but
without experimentation theory is sterile. It is important to follow up hints
given by theory, for one of the great lessons of the history of physics in
the 20th century is that respect for great principles pays rich dividends.
However, it is also important that the prepared mind—and the prepared
apparatus!—go where no one has gone before and explore. The discoveries
recounted in Chapters 6 and 8 by Martin Perl, William Kirk and Leon Lederman
did not result from treasure hunts inspired by theoretical models. An
important motivation for the Super Collider is precisely this: to take a
significant step beyond the present confines and to learn what is there. The
progress of our science demands it.